CD1d molecules present lipid and glycolipid antigens to a unique subset of T cells with important immunoregulatory properties, the NKT cells. Most of the NKT cells in mice use an invariant TCR a-chain, with Va14-Ja18 rearrangement, paired with a limited number of V[unreadable] chains. Another population of CD1d-reactive T cells also exists that expresses a more diverse TCR repertoire, but it remains poorly characterized. While the mouse genome contains two genes that encode for CD1d proteins, only one of them, CD1d1, appear important to the biology of NKT cells. Our preliminary results establish that some iNKT cells selected on CD1d2 molecules are in fact present in the tissues of CD1d1-/- mice. CD1d1 and CD1d2 gene products differ by 17 amino acids, with 16 of them located in the a1 and a2 domains of the protein that could therefore influence antigen presentation and/or recognition. These results raise the possibility that CD1d2 molecules might play a unique role in NKT cell biology. We propose to further characterize CD1d2-selected iNKT cells and to test whether these cells develop in a wildtype environment. PUBLIC HEALTH RELEVANCE: Natural Killer T cells (NKT cells) play an important role in regulating a variety of immune responses. Several major advances have been made in the recent years regarding how these cells develop, recognize their antigens and influence immune responses. Yet, our finding that some iNKT cells continue to develop even in absence of CD1d1 molecule expression challenges the current dogma regarding the development of these cells. The proposed studies aim at further characterizing phenotypically and functionally the iNKT cells that develop in CD1d1-deficient mice. An understanding of iNKT cell development and homeostasis is of fundamental importance to understand the function of these cells so that they can be exploited for therapeutic usage to improve human health.